In vitro calibration of an optical catheter, which may or may not include calibration of the associated instrumentation, is often accomplished with a calibration element of known optical properties placed over the distal end of the catheter tube. Light propagated through transmitting optical fibers in the catheter tube returns from the calibration element through receiving optical fibers in the catheter to suitable instrumentation for measuring and processing the optical signal. The measurements taken provide an optical characterization of the catheter and instrumentation which is used to quantify subsequent measurements taken of a sample under examination.
In calibrating the catheter, it is important that the end portion of the catheter tube be retained in a preferred proximity with the calibration element, and that this be done in a sterile environment while enabling a convenient, repeatable calibration prior to catheter use. Existing devices intended to accomplish this have certain drawbacks which need to be overcome.
For example, U.S. Pat. No. 4,322,164 to Shaw, et al., describes a box that is sealed with the catheter in a dual-envelope sterilizable package so that the end portion of the catheter is located in the box. In order to calibrate the catheter, the box is actuated by pressing a trigger mechanism through the package wrapper, and this causes a resilient holder to grip the catheter tube as a spring drives the calibration element against the catheter tip. Thus, the end portion of the catheter is placed and retained against the calibration element for calibration purposes, but only with a relatively complicated and expensive mechanical device.
U.S. Pat. No. 4,650,327 to Ogi discloses a calibrating device including a tube having a reference block therein which is spring loaded into compliant engagement with the distal end of the optical catheter. A releasable strap tightly secures the catheter to the calibration device. The packaged catheter can then be calibrated by removing the proximal end from the sealed package and connecting it to a processor for performing the calibration operation. Again, the catheter is retained against the calibration element with a relatively complicated and expensive mechanical device.
The reference block in Ogi and Shaw is described as a solid cylindrical element formed of a silicone resin, having a plurality of tiny particles scattered throughout its mass to provide scattering and reflecting surfaces for the light beams transmitted by the catheter. Ogi states that the particles should have dimensions within the range of from about 0.02 to about 2.0 microns.
Ogi states that the mass should be translucent and compliant at the surface so that it will yield when pressed against the rigid surface of the catheter, thereby insuring a snug fit. Shaw states that the solid mass should be substantially transparent, compliant at the surface and noncompressible. Shaw states that for measuring oxygen saturation of the blood, the particles may be titanium dioxide but that other light-scattering particles such as oxides, sulfates and carbonates of magnesium, barium and calcium or the like may also be used. See Column 3, line 67 to Column 4, line 41 of Shaw and Column 3, line 51 to Column 4, line 9 of Ogi.
U.S. Pat. No. 4,050,450 to Polanyi et al., discloses a calibration device described as a generally tubular reflecting member aligned with and adjacent to the distal end of the catheter. The reflecting member may be vinyl tubing or the like which may be removably or fixably positioned about the distal end of the catheter to reflect light directed thereon from the catheter when in air or a clear sterile solution for calibration. Polanyi states that while a variety of tubing materials and coloration are satisfactory a white-pigmented, flexible vinyl tubing is preferred. However, since the calibration element or tube is optically open at its distal end, the device is not immune to ambient light.
U.S. Pat. No. 4,744,656 to Moran et al., discloses a calibration device, referred to as a boot, into which the catheter tip is placed and held gently by a detent formed within the cavity of the boot. A calibration substance faces the tip in a mechanically and optically standardized calibration relationship to reflect light from within the catheter back into the catheter. The calibration substance is held in constant, precise contact with the tip by close fit between the tip and the precision-molded internal surfaces of the cavity. The calibration substance is preferably a homogeneous suspension of reflecting particles in a translucent or transparent polymer. The boot is preferably injection molded from the calibration substance, except for a rigid opaque outer skin. The specification describes the base material as a substantially transparent, medical-grade moldable high-strength silicone. The filler is described as silica-free magnesium oxide, obtained as a white powder with a maximum particle size of roughly 1/30th of a micron. See Column 10, line 58-64.
The subject matter of the present application relates to the subject matter of commonly assigned U.S. Pat. No. 4,796,633 to Zwirkoski, entitled Method and Apparatus for In Vitro Calibration of Oxygen Saturation Monitor, and commonly assigned U.S. patent application, Ser. No. 942,356, entitled Catheter Calibration Device, in the name of Manska, et al. The disclosures of the Zwirkoski patent and the Manska patent application are incorporated herein by reference in their entirety.
Specifically, Zwirkoski discloses a calibration element comprising an elongated tubular wall open at one end with an integral end wall closing the other end. The end wall defines a curved cavity opening toward the open end of the tubular wall. The calibration element is adapted to receive a light guide through the tubular wall and into the cavity. The cross-section of the cavity is progressively reduced distally to limit the extent to which the light guide can be advanced into the cavity so that the end face of the light guide is spaced from the inner surface of the end wall to define a hemispherical gap. The end wall and the gap are adapted to return a known ratio of the light directed into the gap from the end face of the light guide.
The catheter calibration device disclosed in the Manska application includes the calibration element of Zwirkoski and a clamp member of resiliently deformable material with which to hold the catheter tube and retain the end portion of the tube in the cavity of the calibration element. A retainer member is also provided to retain the clamp member in generally fixed proximity to the open end of the cavity of the calibration element. A light-blocking cap encases the optically active portion of the calibration element.
The Zwirkoski calibration element has a spherical inner surface with relatively thin walls approximately 0.045 inches thick and requires the use of an opaque optical barrier (styrene backing) to obtain the correct optical ratio and to prevent ambient light from being received by the optical fiber in the catheter. Without the opaque barrier, the back scattered ratio is out of the acceptable range.
In its simplest form, the mathematical representation of the optical signals in a particulate media is the Beer-Lambert-Bauger equation: I=I.sub.o .times.exp (Q.sub.ext .times.N.times.d) where I=transmitted light intensity, I.sub.o =incident light intensity, Q.sub.ext =extinction coefficient at a specific wavelength, N=number of particles per unit volume, and d=path length through the particulate medium.
The optically active part of the Zwirkoski calibration element is sufficiently thin to allow the optical signal I.sub.o to be transmitted through the walls of the calibration device reflect off the opaque optical barrier and be measured as part of the return signal by the receiving fiber. The wall thickness of the calibrator is variable due to manufacturing tolerances. The air gap between the calibrator and opaque optical barrier is variable. The shape of the catheter tip and the polishing depth also varies from catheter to catheter thereby varying the length of the air gap between the end face of the catheter and the inner surface of the calibrator. Finally, the opaque optical barrier is not controlled for its optical absorption and reflectance properties. Thus, the path length d in Beer's equation is variable resulting in undesirable variation in the reference signal.
Since the inner optical surface is smooth, some of the transmitted light is reflected from the surface and is returned to the receiving fiber without being acted upon by the scattering and absorbing materials in the calibration device. This is known as specular reflection. Specular reflection is only of concern when the sending and receiving fibers are parallel to each other and I is the backscattered light. Specular reflection is an undesirable signal for calibration purposes since the signal is not acted upon by the scattering and absorbing materials within the calibration device.
The amount of specular reflection received by the light guide is determined by the distance of the reflecting surface from the end face of the light guide and the shape of the reflecting surface. The spherical shape of the inner optical surface in the Zwirkoski calibrator functions to focus the specular reflection at the catheter tip, thereby aggravating the effects of specular reflection. Further, the spherical inner surface of the Zwirkoski calibrator moves the end face of the catheter away from the optical surface of the calibrator. The increased distance between the reflecting surface and the receiving fiber increases the amount of specular reflection.